1. Field of the Invention
This invention relates to a mold for the production of a silica crucible. Particularly, the invention is directed to advantageously reduce raw material cost by devising an upper form of an inner wall in an opening portion of the mold.
2. Description of the Related Art
Recently, the use of a silicon wafer as a substrate for semiconductor devices has increased rapidly. Such a silicon wafer is usually produced by forming a silicon single crystal ingot and then slicing it.
Such a silicon single crystal ingot is generally produced, for example, by a pulling method such as the CZ method, or the like. Also, a silica crucible is used for pulling the silicon single crystal.
A typical method of producing a silica crucible is known as the rotating mold method. The rotating mold method is a method wherein silica or quartz powder is attached to an inner wall of a rotating mold, that is, a bottom surface and a side surface of the mold and then fused by heating (e.g., by arc heating) to produce a silica crucible.
Recently, the silicon single crystal ingot is desired to have a larger diameter in association with the rapid increase in the demand for silicon wafers.
When such a large-diameter silicon single crystal ingot is produced by the pulling method, the silica crucible to be used is also required to have a larger diameter.
In order to produce the silicon single crystal ingot by the pulling method, it is usually required to use a silica crucible having a diameter corresponding to about three times the diameter of the ingot.
When the silica crucible is produced by the above-mentioned rotating mold method, a portion having a small outer diameter and a thin thickness (hereinafter referred to as a small-diameter thinned portion) occurs in an upper part of the silica crucible, and hence it is required to remove the small-diameter thinned portion by cutting.
When the silica crucible is heated to a high temperature in a step for filling and melting Si in the silica crucible to pull a Si single crystal, the viscosity of silica is reduced as the temperature rises and hence the silica crucible is easily deformed. Especially when the upper part of the silica crucible is the small-diameter thinned portion, it is easily deformed.
A cause of forming such a small-diameter thinned portion is that heat easily escapes at the opening portion of the mold and the silica or quartz powder is not completely melted by arc heating, and hence the outer diameter at the upper part of the silica crucible becomes small and the thickness thereof becomes thin. The formation of such a small-diameter thinned portion is unavoidable in the production process.
Therefore, when the silica crucible is produced by the rotating mold method, by anticipating the formation of the small-diameter thinned portion is produced a silica crucible having a crucible height higher by the small-diameter thinned portion than a height of a product specification and then the small-diameter thinned portion is removed by cutting to obtain a product.
Also, silica crucibles having different heights may be produced in the same mold. Although there is no problem in the production of a silica crucible having a higher height, a portion to be removed by cutting becomes large in the production of a silica crucible having a low height. In order to solve this problem, a mold for exclusive use in a product having a low height may be provided, but when the number of silica crucibles produced is small, or the like, there is a disadvantage that the production cost of a new mold and related costs become significant.
As described above, when the silica crucible is produced by the rotating mold method, the formation of the small-diameter thinned portion is unavoidable, so that a silica crucible having a crucible height higher by the small-diameter thinned portion than a height of a product specification is produced and then the small-diameter thinned portion is removed by cutting to obtain a product.
However, the silica crucible is recently required to have a large diameter with the increase of the diameter of the silicon single crystal ingot as described above. When the diameter of the crucible is made larger, a portion obliged to be removed by cutting is also increased due to the small-diameter thinning, which causes a serious problem in terms of the material cost, and hence the production cost.
For example, if it is intended to produce a crucible having an outer diameter of 18 inches and a thickness of 8 mm, about 0.3 kg of silica or quartz powder as an absolute amount is used extra per a cutting-removing height of 10 mm.
On the other hand, if it is intended to produce a large-size silica crucible having an outer diameter of 32 inches, the thickness increases to approximately 15 mm, and hence about 1.0 kg of silica or quartz powder as an absolute amount is used extra per cutting-removing height of 10 mm.
In the production of a large-size silica crucible, therefore, the wasted amount of silica or quartz powder used per the cutting-removing height in the production of one crucible increases to three to four times that of producing a small-size silica crucible.